In recent years, polycrystalline silicon, which has high mobility, has been expected to be used as channel silicon in a semiconductor memory (three-dimensional memory) device in which thin film transistors (TFT) and memory cells are three-dimensionally arranged.
To allow such a polycrystalline silicon film to be formed so as to have a smooth morphology and to appropriately cover steps, first, amorphous silicon is deposited at a low temperature of between 450° C. and 550° C. The amorphous silicon offers high resistance and thus needs to be made polycrystalline to offer reduced resistance. Thus, the amorphous silicon is thermally treated at a high temperature of at least 900° C. so as to be crystallized. However, when the amorphous silicon is thermally treated at such a high temperature, nuclei are formed in the amorphous silicon. The amorphous silicon thus has a small grain size and different crystal shapes. As a result, the number of interfaces (grain boundaries) between crystals increases and the polycrystalline silicon film has a roughened morphology. This disadvantageously prevents the resistance from being sufficiently reduced.
Furthermore, disadvantageously, the thermal treatment at high temperatures may, for example, reduce a gate dielectric strength voltage or increase a junction leakage current in peripheral transistors.
As a method for crystallizing the amorphous silicon to increase the grain size, a method has been proposed in which an amorphous Si film is crystallized by growing the amorphous silicon in a solid phase using crystals Ge contacted with the amorphous silicon, as nuclei (see, for example, V. Subramanian et al., “High-performance Germanium-seeded Laterally Crystallized TFT's For Vertical Device Integration” IEEE Transactions On Electron Devices, Vol. 45, No. 9, September 1998, pp. 1934-1939). In this case, SiO2 is formed on amorphous silicon with a thickness of 100 nm, and a 1 μm hole is formed in the amorphous silicon. Then, Ge is formed in the hole between 450° C. and 500° C. so that the amorphous silicon is crystallized using Ge as nuclei. A crystallization temperature for the silicon is between 500° C. and 550° C. Thereafter, Ge is etched away with H2SO4/H2O2. Furthermore, SiO2 is etched away with HF to obtain a polycrystalline Si layer. However, this step is complicated. The crystallization of amorphous silicon requires several hours at 600° C. Thus, if the temperature is set to a smaller value of between 500° C. and 550° C., the crystallization is expected to require several tens of hours, though this may depend on the interval of the Ge nuclei. Hence, throughput is low, disadvantageously leading to an increase in manufacturing costs.
Furthermore, a method has been proposed which comprises forming island-shaped Ge (see K. Yasutake et al., “Size and Density Control of Crystalline Ge Islands on Glass Substrates by oxygen Etching” Japan Journal of Applied Physics, Vol. 43, No. 12A, 2004, pp. L1552-L1554), then depositing amorphous silicon, and thereafter crystallizing the amorphous silicon by thermal treatment to obtain silicon with a large grain size (see C. Yoshimoto et al., “Formation of Polycrystalline Si Thin Films Using Nanocrystalline Ge Nuclei”, IEICE Technical Report SDM2008, April 2008, pp. 89-93). To form island-shaped Ge, first, Ge nuclei with an average grain size of 89 nm and a density of about 108/cm2 are formed by depositing an amorphous Ge film to a thickness of 50 nm and carrying out vacuum annealing at 500° C. for 2 hours and oxygen etching at 400° C. for 3 hours. Thereafter, an amorphous silicon film is deposited by electron beam evaporation or plasma CVD and then thermally treated between 480° C. and 620° C. Thus, the amorphous silicon film changes to a polycrystalline silicon film with silicon with a large grain size. However, also in this case, the crystallization of the amorphous silicon requires a long time: about 2 hours at 580° C., about 12 hours at 540° C., and at least 20 hours at 500° C. or 480° C. At the low temperatures, the throughput is low, disadvantageously increasing manufacturing costs. Furthermore, since the crystal Ge nucleus has a grain size of several tens of nm, the amorphous silicon has a film thickness of at least 100 nm.
In general, the crystallization temperature tends to increase with decreasing film thickness. Thus, crystallization of amorphous silicon with a film thickness of at most 10 nm is expected to require a further longer crystallization time than crystallization of amorphous silicon with a film thickness of 100 nm. A further lower throughput results from crystallization of a thin amorphous silicon film with a film thickness of at most 10 nm, disadvantageously leading to an increase in manufacturing costs.
Thus, the conventional art has difficulty crystallizing amorphous silicon at low temperatures in a short time.